Revert "MT#5869 Add mirror ftp.at.sipwise.com for virtualbox-guest-additions-iso"

This reverts commit 84daf10c21.
There's no need for usage of ftp.at.debian.org any longer, as we
have our very own Debian mirror (see MT#6171).

deployment.sh

@@ -1772,11 +1772,8 @@ vagrant_configuration() {
     echo "/usr/share/virtualbox/VBoxGuestAdditions.iso exists already"
   else
     echo "/usr/share/virtualbox/VBoxGuestAdditions.iso does not exist, installing virtualbox-guest-additions-iso"
-    # Add mirror ftp.at.sipwise.com, see MT#5869
-    echo "deb http://ftp.at.debian.org/debian/ testing main contrib non-free" > /etc/apt/sources.list.d/austrian-debian.list
     apt-get update
     apt-get -y --no-install-recommends install virtualbox-guest-additions-iso
-    rm -rf /etc/apt/sources.list.d/austrian-debian.list
   fi
 
   if [ ! -r "$isofile" ] ; then